2nd mobile communication refers to transmission and reception of voice through mobile communication, which includes CMDA, GSM, and the like. GPRS, advancing from the GSM, has been proposed to provide a packet switched data service based on the GSM system.
3rd generation mobile communication allows for transmission and reception of image and data, as well as voice, and 3GPP (Third Generation Partnership Project) has developed a mobile communication system (IMT-2000) technique and adopts WCDMA as a radio access technology (RAT). The combination of the IMT-2000 technique and the radio access technology (RAT), e.g., WCDMA, is called a UMTS (Universal Mobile Telecommunication System). UTRAN is an acronym of UMTS Terrestrial Radio Access Network.
3rd generation mobile communication is evolving to 4th mobile communication.
The 4th mobile communication technique includes a long-term evolution network (LTE) technique under standardization by 3GPP and an IEEE 802.16 technique under standardization by IEEE. The LTE uses a term of E-UTRAN (Evolved-UTRAN).
The 4th mobile communication technique has introduced OFDM (Orthogonal Frequency Division Multiplexing)/OFDMA (Orthogonal Frequency Division Multiple Access). OFDM uses a plurality of orthogonal subcarriers. OFDM uses orthogonality between IFFT (Inverse Fast Fourier Transform0 and FFT (Fast Fourier Transform). A transmitter performs IFFT on data and then transmits the same. A receiver performs FFT on received signal to restore the original data. The transmitter uses IFFT in order to combine a plurality of subcarriers and the receiver uses corresponding FFT in order to split the multiple subcarriers.
Meanwhile, in the 3rd and 4th mobile communication system, attempts for increasing cell capacity continue to support high capacity services such as multimedia contents, streaming, and the like, and bi-directional services.
In order to increase cell capacity, there has been an approach of using a high frequency band and reducing a cell radius. The use of a cell, such as a pico cell, or the like, having a small cell radius allows for the use of a higher frequency band than that used for the existing cellular system, having the advantages that more information can be delivered. However, because a larger number of base stations must be necessarily installed in the same area, much cost incurs.
Thus, recently, a femto cell has been proposed as one of approaches of increasing cell capacity by using small cells.
Femto cell refers to provision of a small-scale radio environment by installing a base station using small power in indoor spaces such as homes, offices, and the like. The femto cell is expected to improve an indoor service available area and increase capacity to thus enhance quality of service (QoS), and also expected to completely settle the next generation mobile communication system by providing data services.
For such a femto cell, standardization is ongoing in the name of Home eNodeB by 3GPP WCDMA and LTE group, and 3GPP2 is also actively studying femto cell.
Various structures as illustrated in FIGS. 1 and 2 have been proposed in order to implement such a femto cell in the existing mobile communication network.
First, FIG. 1 illustrates an example of a network structure based on femto cells according to the related art.
As shown in FIG. 1, a macro base station (M-BS) serving a wider area and a plurality of femto base stations (f-BSs) installed based on users.
The f-BSs are connected with a femto cell network controller (FNC) through the Internet so as to be under the control of the FNC, and provide services to users.
A terminal measures signals of neighboring cells and delivers the measured signal values to its f-BS, and the f-BS recognizes and administers the presence of neighboring cells based on the received signal values. Also, the f-BSs exchange information through a direct link or an indirect link through the FNC. The f-BSs and the M-BS transmit and receive information through the FNC, an RNC (Radio Network Controller) or through an MME (Mobility Management Entity) that controls the f-BSs in a mobile communication network.
FIG. 2 illustrates another example of a femto cell-based network structure according to the related art.
As shown in FIG. 2, f-BSs exchange information through a direct link or an MME, unlike the case illustrated in FIG. 1. Also, the M-BS and the f-BSs exchange through MMEs.
Meanwhile, recently, a service called FMC (Fixed Mobile Convergence) has been introduced. The FMC technique provides a cellular-based radio access technology (RAT), for example, a CDMA, TDMA, IMT-2000 (e.g., CDMA2000, W-CDMA), and LTE-based access scheme and a unlicensed frequency-based radio access technology (RAT) (e.g., Unlicensed Mobile Access, Bluetooth or Wi-Fi) to a terminal. To this end, the terminal must support a dual-mode.
Thus, when the dual-mode terminal comes close to a unlicensed frequency-based radio base station, the dual-mode terminal is allowed to use the same service as that in a cellular-based RAT through the unlicensed frequency-based RAT. For example, when the dual-mode terminal enters an area of a small radio base station using the unlicensed frequency-based RAT while it is communicating with the cellular mobile communication base station (i.e., the M-BS), it performs handover according to the unlicensed frequency-based RAT of the small radio base station to continue communication of high band seamlessly. In other words, as a terminal performs seamless handover between base stations, the dual-mode terminal is able to perform seamless handover between the cellular-based RAT and the unlicensed frequency-based RAT.
There have been attempts of applying such an FMC technique to a femto base station. Namely, among them is an attempt of combining the FMC communication technique along with cellular mobile communication to the femto base station.
FIG. 3 illustrates an example of combining the femto base station and the FMC technique.
As shown in FIG. 3, an M-BS 30 and an f-BS 20 are illustrated. The M-BS 30 is a general BS, but in this case, because the size of its cell radius is larger than the f-BS, the general BS is called the M-BS.
The f-BS 20 provides both a first RAT such as cellular communication and a second RAT based on an unlicensed frequency. Thus, when the terminal is located within the coverage of the f-BS 20, the terminal can be provided with a service through the unlicensed frequency-based RAT.
However, the related art does not provide a substantial procedure as to which of the first RAT such as the cellular communication or the second RAT such as unlicensed frequency based RAT provided by the target f-BS 20 the terminal 20 is to select when the terminal 10 enters the coverage of the target f-BS 20.
Thus, the related art terminal 10 preferentially selects the first RAT such as cellular communication provided by the target f-BS 20 for its accessing, which results in a failure of utilization of the FMC technique.
Also, if a femto BS supports multiple RATs, the femto BS could deactivate a specific RAT which is not used among the multiple RATs. In such situation, the terminal could not use the specific RAT to connect to the femto BS since the technique to allow the terminal to use the deactivated RAT is not provided.